This invention relates to electric motor control, and in particular to pulse width modulation (PWM) control of multiple phase brushless motors.
Control systems for electric motors need to measure the current through the windings or phases of the motor and this can either be done by means of separate current sensors for all of the phases, or by means of a single current sensor that is placed in the circuit so as to measure the total instantaneous current flowing between a D.C. power supply and the bridge circuit and motor combination. In a single current sensor system, the multiple motor phase currents are derived by offsetting the PWM patterns of the switches which apply the required voltage to each phase, and sampling the current sensor at appropriate points. Under certain configurations this can place constraints on the PWM pattern that limit the maximum fundamental phase voltage that can be produced.